Steppers and VFD - Chemeketa Community Collegefaculty.chemeketa.edu/csekafet/ELT291/18.pdf ·...

Industrial Motors

• DC Motors• AC Motors• Three Phase Motors• Specialty Motors

– Variable Frequency Drives – Stepper Motors

But first…..Servos!

Servos can be AC or DC but they do one thing:

Sense the output position and adjust the input source to compensate for the difference.

Variable Frequency Drives

Consider VFDs for pumps and fans where flow varies

Variable frequency drives (VFDs), a type of variable speed drive, are motor controllers

that vary the speed of squirrel cage induction motors


VFDs save substantial energy when applied to variable-torque loads, and result in

reductions in electricity bills in most facilities


These energy savings are possible with variable-torque loads, such as fans and

pumps, because torque varies as the square of speed, and horsepower varies as the cube

of speed. For example, if fan speed is reduced by 20%, motor horsepower (and energy consumption) is reduced by 50%.


VFDs generate variable voltage and frequency output in the proper volts/hertz ratio for the

motors from the fixed utility-supplied power


VFDs can be retrofitted into existing motor systems, and can operate both standard and high-efficiency motors ranging in size from

1/3 HP to several thousand HP


Unlike mechanical or hydraulic motor controllers, they can be located remotely and do not require mechanical coupling

between the motor and the load


Applications:

centrifugal fanspumps (centrifugal, propeller, turbine)

agitators axial compressors


Three major VFD designs

1. Pulse Width Modulation (PWM)2. Current Source Inverter (CSI)

3. Variable Voltage Inverter (VVI)

Flux Vector PWM


Pulse Width Modulation (PWM)

PWM outputs emulate sinusoidal power waves by varying the width of pulses in

each half cycle



Advantages of PWMs are low harmonic motor heating, excellent input displacement

power factor, high efficiencies at 92% to 96%, and ability to control multiple motor

systems with a single drive.



The dominant VFD design in the 1/2 HP to 500 HP range because of its reliability,

affordability and availability


Current Source Inverter (CSI)

Quite reliable due to their inherent current-limiting characteristics and simple circuitry



CSIs have regenerative power capabilities, meaning that CSI drives can reverse the power flow back from the motor through

the drive



CSIs "reflect" large amounts of power harmonics back to the source, have poor input power factors, and produce jerky motor operations (cogging) at very low

speeds



CSIs are typically used for large(over 300 HP) induction and synchronous

motors.


Voltage Source Inverter (VSI)

Similar to CSI designs, but VSIs generate variable-frequency outputs to motors by

regulating voltage rather than current


Voltage Source Inverter (VSI)

Harmonics, power factor and coggingat low frequencies can be problems.


VFD’s should be properly installed to avoid damage to their electronics. This includes proper grounding, mounting, connection,

voltage, and cooling.


[1] Installing VFDs intended for wall mounting as free standing units will interfere with the "chimney effect"

cooling of the heat sink. Always install wall-mounted units against a smooth,

flat, vertical surface or install a piece of plywood or sheet metal to create the

required cooling channels.


[2] Ensure that the power voltage supplied to VFDs is stable within plus or minus

10% to prevent tripping faults.


[3] Motors operating at low speeds can suffer from reduced cooling. For

maximum motor protection on motors to be run at low speeds, install thermal sensors that interlock with the VFD

control circuit. Standard motor protection responds only to over-current

conditions.


[4] Speed control wiring, which is often 4mA to 20mA or 0 VDC to 5 VDC, should be separated from other wiring to avoid erratic behavior. Parallel runs of 115V

and 24V control wiring may cause problems.


Precautions for specifying, installing and operating VFDs are numerous. Improper

installation and start-up accounts for 50% of VFD failures.


[1] Use the VFD start-up sheet to guide the initialization check prior to energizing

the VFD for the first time.[2] Corrosive environments, humidity above 95%, ambient air temperatures

exceeding 40°C (104°F), and conditions where condensation occurs may damage

VFDs


[3] If a VFD is started when the load is already spinning, the VFD will try to pull

the motor down to a low, soft-start frequency. This can result in high current and a trip unless special VFDs are used.


[4] Switching from grid power to emergency power while the VFD is

running is not possible with most types of VFD’s. If power switching is anticipated,

include this capability in the specification.


[5] If a motor always operates at rated load, a VFD will increase power use, due

to electrical losses in the VFD.

Stepper Motors

Applications

Stepper Motor Characteristics

Voltage

Resistance

Degrees per step

Degrees per Step

• Figured out by– Looking at the datasheet– Looking at the motor specifications– Turning the motor by hand and dividing the

number of steps into 360 to determine the degrees per step

– Divide the steps per revolution on the motor casing into 360

TYPES OF STEPPER MOTORS

Two Basic Types:

• Permanent Magnet

• Variable Reluctance

Permanent Magnet

Sub Categories:

Unipolar Stepper Motors

Bipolar Stepper Motors

Variable Reluctance Stepper

Unipolar Stepper MotorsThe most common stepper is the four-coil

unipolar variety, a typical coil format being the one shown below:


• These are called unipolar because they require only that their coils be driven on and off.


The normal stepping sequence for four-coilunipolar steppers is shown in the following diagram:

Typical Motor Driver

Full Phase and Half Phase


Bipolar motors are known for their excellent size/torque ratio, and provide more torque for their size than unipolarmotors


Designed with separate coils that need to be driven in either direction (the polarity needs to be reversed during operation) for proper stepping to occur


Bipolar stepper motors use the same binary drive pattern as a unipolar motor, only the '0' and '1' signals correspond to the polarity of the voltage applied to the coils, not simply 'on-off' signals.

Typical H-Bridge Circuit Driver

Variable Reluctance Stepper Motors

Referred as Hybrid motors.

Simplest of the stepper motors to control


• Bipolar steppers require that the polarity of power to the coils be reversed

• Their drive sequence is simply to energize each of the windings in order, one after the other


• It will often have only one lead, which is the common lead for all the other leads.

• Feels like a DC motor when the shaft is spun by hand; it turns freely and you cannot feel the steps.


• This type of stepper motor is not permanently magnetized like its unipolarand bipolar counterparts.

Steppers

• RotationalGears

Pulleys

• LinearX-Y Tables

Steppers and VFD - Chemeketa Community Collegefaculty.chemeketa.edu/csekafet/ELT291/18.pdf ·...

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